Verizon came in first place with an average download speed of 1,940 Kbps, as reported by 856 participants. T-Mobile’s average rate was 1,793 Kbps with 1,189 reported T-Mobile users. Third was Sprint with 1,598 Kbps, based on data from 1,570 users. In dead last was AT&T with an average of 901 Kbps — but an overwhelmingly large user sample of 8,153 test takers.

The debate over which carrier is best has been around for as long as there have been cellphones. A common myth is that Verizon’s is the most reliable and fast, while AT&T’s is the worst. These claims have been difficult to verify because of the variable nature of wireless technology. But Wired.com gave it a try on U.S. phones anyway.

To gather the data, Wired.com asked U.S. smartphone owners to test their 3G download speeds by running a custom web page that involved downloading an image. Then, we asked participants to manually plot their data on an interactive map. For a second data set, our test developer Ben Reubenstein automatically tracked download speeds using the test page.

After analyzing both data sets, Wired.com opted to use Reubenstein’s automatically reported results, because the information was more complete, comprehensive and reliable.

We acknowledge our speed test is not scientific; we view it as a general barometer that gauges the performance of 3G networks in the United States. With that said, we note a few caveats:

We realize that the substantially larger amount of AT&T test takers may unfairly contribute to an overall lower download speed — so take this considerably lower result with a nugget of salt.

The test results rely heavily on good faith that participants were being honest (and not, for instance, running the test over Wi-Fi). We did, however, remove any bogus, impossibly high results, as well as incomplete data fields.

Software, processors, memory and other factors will obviously affect a cellphone’s overall download speeds. Smartphones with faster CPUs, more RAM and so on will clearly give the network an advantage. But then again, these test results should give you a general idea of the network performance you should expect not only on these networks, but also the phones they carry.

Interestingly, our results appear to coincide with a similar test conducted by PC World in May. PC World’s test involved working with Novarum Inc., a wireless consultant firm, who performed a 3G stress test from 283 locations in a day. PC World’s testing also found Verizon was fastest, with Sprint and AT&T coming in second and third, respectively. (T-Mobile was not included in the PC World test.)

With that said, we thank Reubenstein for coding our test, and we also thank our fellow friends in the blog community for helping spread word of the study to attract participants. We invite readers to take a look at our spreedsheet containing the results [csv] if they wish to perform their own analysis.